In creating a multiple layer (level) semiconductor device on a semiconductor wafer, each layer making up the device may be subjected to one or more deposition processes, for example using chemical vapor deposition (CVD) or physical vapor deposition (PVD), and usually including one or more dry etching processes. A critical condition in semiconductor manufacturing is the absence of particulate on the wafer processing surface, since microscopic particles may interfere with and adversely affect subsequent processing steps leading to device degradation and ultimately semiconductor wafer rejection.
While the wafer cleaning process has been always been a critical step in the semiconductor wafer manufacturing process, ultraclean wafers are becoming even more critical to device integrity. For example, as semiconductor feature sizes decrease, the detrimental affect of particulate contamination increases, requiring removal of ever smaller particles. For example, particles as small as 5 nm may be unacceptable in many semiconductor manufacturing processes. Further, as the number of device layers increase, for example to 5 to 8 layers, there is a corresponding increase in the number of cleaning steps and the potential for device degradation caused by particulate contamination. To adequately meet requirements for ultraclean wafers in ULSI and VLSI the wafer surface must be essentially free of contaminating particles.
Another factor in modern processing technology that increases the incidence of particle contamination is the deposition of carbon doped oxides as IMD layers to achieve dielectric constants of less than 3.0. The IMD layers are typically deposited by a plasma enhanced CVD (PECVD), low pressure CVD (LPCVD), or high density plasma CVD (HDP-CVD). In these processes, a degree of sputtering occurs as the layer of material is deposited, causing a higher degree of particulate contamination as the deposition time increases. In addition, PVD processes are typically used to deposit films of metal, for example barrier/adhesion layers within anisotropically etched features, or for metal filling an anisotropically etched feature. PVD processes tend to coat the inner surfaces of the processing chamber with a metal film, flaking off to contaminate a wafer process surface as the metal film increases in thickness and is subjected to cyclic thermal stresses. Other processes that frequently result in particulate contamination include plasma etching processes where a photoresist layer is etched away during an ashing process. Over time, the buildup of ashing residue within a plasma etching chamber increases the probability that a semiconductor wafer will become contaminated by particulates.
Particulate contamination may cause ‘killer defects’ resulting in integrated circuit opens or shorts by occluding a portion of a circuit or providing a shorting path between two conductive lines of a circuit.
Common processes in use for cleaning wafers include cleaning solutions based on hydrogen peroxide. At high pH values (basic) organic contamination and oxidizable particles, are removed by an oxidation process. At low pH (acidic) metal contamination is desorbed from the wafer surface by forming a soluble complex.
Typically, to reduce processing times and increase throughput, in prior at processes, ex-situ cleaning processes are performed following particle generating processes such as plasma etching or PECVD film deposition. For example, common particle removal mechanisms which may be exploited, depending on the particle and how it adheres to the surface, include dissolution, oxidizing degradation and dissolution, physical removal by etching, and electrical repulsion between a particle and the wafer surface.
Standard wafer cleaning processes have included mechanical scrubbing and ultrasonic methods, for example megasonic agitation of the wafer surface in a cleaning solution or in deionized water to effectuate particulate removal. A shortcoming of mechanical scrubbers includes a demonstrated difficulty in removing particles smaller than about 300 nm. In addition, mechanical scrubbers may produce an unacceptable level of scratching in soft materials. In using a megasonic source of agitation, megasonic transducers operating in a frequency range near about 1 MHz are typically attached to the side or bottom portion of a cleaning tank filled with cleaning solution with the surfaces of the process wafers arranged parallel to the direction of traveling or standing megasonic waves induced at the side of the cleaning tank. The transducer is typically rectangular shaped and integrally attached to the cleaning tank to achieve megasonic cleaning action. A shortcoming of prior art megasonic cleaning processes is the relatively low level of cavitation action produced by megasonic transducer arrangements of the prior art. Under certain conditions the ultrasonic energy also creates cavitation bubbles within the liquid and subsequently collapse where the sound pressure exceeds the liquid vapor pressure. When the cavitation bubbles collapse, energy is released causing turbulent flow which can dislodge particles adhering to the wafer surface. Typical stegasonic transducers of the prior art have not sufficiently coupled ultrasonic energy into the cleaning solution to create a sufficiently high density of cavitation bubbles to achieve primarily cavitation collapse induced particulate cleaning. As a result, prior art processes have not been fully successful in removing smaller particles from wafer process surfaces, particularly those smaller than about 0.3 microns.
Another shortcoming of prior art cleaning processes, for example megasonic cleaning processes, is the tendency for the removed particles to reattach to the wafer surface. For example, following treatment of a large number of wafers in megasonic cleaners of the prior art, the cleaning solution must frequently be replaced to avoid particulate recontamination of process wafers.
There is therefore a need in the semiconductor wafer processing art to develop an ultrasonic cleaning system and method driven primarily by cavitation whereby particulate contamination is effectively removed from a process wafer surface while avoiding particulate recontamination.
It is therefore an object of the invention to provide an ultrasonic cleaning system and method driven primarily by cavitation whereby particulate contamination is effectively removed from a process wafer surface while avoiding particulate recontamination, in addition to overcoming other shortcomings and deficiencies of the prior art.